1. Field of the Invention
This invention relates to an anti-skid control system for achieving satisfactory rotational behavior of wheels of a motor vehicle during braking operation. More particularly, this invention pertains to an anti-skid control system which is designed so that brake fluid pressure reduced to prevent the wheels from being locked can be increased in a desirable manner.
Furthermore, the present invention is also directed to an anti-skid control system capable of preventing anti-skid wheel locking which tends to occur when the coefficient of friction between the wheels and the road surface is very low, i.e., when the motor vehicle is running on a road surface with a very low coefficient of friction.
2. Description of the Prior Art
There have heretofore been proposed a variety of anti-skid control systems wherein during braking operation, the brake fluid pressure is reduced to prevent occurrence of such an undesirable wheel braking condition as to cause wheel lock, and the brake fluid pressure thus reduced is now increased to avoid an undesirable extension of the brake stopping distance. Among such conventional anti-skid control systems is one wherein different pressure buildup rates are stored; a desired one of the stored pressure buildup rates is selected; and brake fluid pressure is increased with the selected buildup rate, for example. As concrete means for effecting the pressure buildup rate selection, a system has also been proposed wherein a solenoid-operated valve adapted to be opened and closed with a relatively high frequency is provided in the brake fluid pressure buildup system; a pulse train generator such as multivibrator is provided in the drive system for the solenoid-operated valve; and the timing with which the aforementioned solenoid-operated valve is driven by a pulse train signal derived from the multivibrator, is controlled on the basis of a signal representing wheel acceleration (British Patent Specification No. 1305430).
However, more satisfactory anti-skid control is hardly achievable with the aforementioned conventional system wherein anti-skid control operation is performed through the opening and closing of a solenoid-operated valve which is driven by a pulse train signal either with a single average pressure buildup rate or with one selected from several different pressure buildup rates. This will also be understood from what is mentioned below. The decrease and recovery of the wheel speed requiring anti-skid control is influenced by various factors such as the conditions of the road surface on which the motor vehicle is running, the coefficient of friction of the road surface, the vehicle running speed, the brake operating condition, the tire surface condition, the internal pressure of the tire tube, and so forth, and the overall effect of these factors is always changing in succession. However, any of the above-described conventional systems fails to respond quickly enough to cope with such a situation. This difficulty becomes most remarkable with brake fluid pressure buildup mode in anti-skid control which requires that delicate balance be achieved between two contradictory requirements, i.e., the requirement that occurrence of re-locking of a wheel be restrained as effectively as possible and the requirement that any undesirable extension of the brake stopping distance be prevented.